Several probe-based instruments have been developed that obtain information from the interaction between its sensing element and a sample being analyzed. At least some of these employ a stylus or tip carried by a lever, such as the lever of a cantilever. The stylus is located at or adjacent the free end of the cantilever and interacts with a sample during instrument operation to obtain information about the sample, such as size, surface contour or topography, shape, roughness, atomic makeup, molecular makeup, and/or other characteristics.
One type of probe-based instrument is a scanning probe microscope (SPM), which can be used to characterize a surface of a sample, even down to the atomic level, by monitoring interaction between a probe and a sample. With SPMs, the tip is typically located at or adjacent the free end of the cantilever of the probe. During operation, relative movement purposefully introduced between the sample and probe enables desired information about the sample to be obtained over a particular region or portion of the sample. During operation, this relative movement is controlled in order to scan the probe and/or sample over a particular region of the sample that is to be analyzed. For instance, where the information obtained relates to the contour of an outer surface of the sample, it can be used to generate a topographic map or the like from which the outer sample surface can be visualized.
An atomic force microscope (AFM) is a very popular type of SPM. A cantilever of a typical AFM probe includes such a tip and is fixed or otherwise extends outwardly at its base from a support or substrate of the probe device. The lever of the cantilever usually is quite small, typically having a width no greater than about fifty microns and a length no greater than about 500 microns (and can be as short as ten microns or less). The tip of the probe usually is quite sharp and can have a radius that is somewhere around three and fifty nanometers in diameter.
During operation, the probe is brought very near to or into contact with a surface of a sample to be examined, and a deflection of the cantilever in response to the interaction of the probe tip by moving either or both of the probe and the sample, with the sample is measured with an extremely sensitive deflection detector. An example of one type of often used deflection detector is an optical lever system, such as described in Hansma et al. U.S. Pat. No. RE 34,489. If desired, some other type of deflection detector, including a deflection detector that employs one or more strain gauges, capacitance sensors, or the like can be used.
The probe is scanned over the sample surface to obtain the desired information about the sample for a particular region or area of the sample. For example, a high-resolution three axis scanner that acts on the sample, support, and/or the probe can be employed. Such a scanner provides the relative motion between the probe and the sample to be carefully controlled, enabling the AFT to measure, for example, the topography or some other surface characteristic of the sample, such as is described in, e.g., Hansma et al. U.S. Pat. No. RE 34,489; Elings et al. U.S. Pat. No. 5,226,801; and Elings et al. U.S. Pat. No. 5,412,980.
AFMs may be designed to operate in a variety of modes, including contact mode and oscillating mode. In contact mode operation, the probe is typically scanned across the surface of the sample while keeping the force of the tip of the probe against the surface of the sample generally constant. This is usually accomplished by moving either the sample or the probe vertically relative to the surface of the sample being analyzed in response to sensed deflection of the cantilever as the probe is scanned horizontally across the surface of the sample. Vertical motion feedback information can be stored, along with other tip-sample interaction feedback information, and used in constructing an image or the like representative of the surface of the sample that corresponds to the sample characteristic being measured, e.g., surface topography.
Depending on the AFM, some AFMs can operate in an oscillating mode, such as TappingMode™ (TappingMode™ is a trademark owned by Veeco Instruments Inc. of Santa Barbara, Calif.). In TappingMode™, the probe is oscillated at or near a resonant frequency of the cantilever. The amplitude or phase of this oscillation is kept constant during scanning using feedback information, which is obtained from tip-sample interaction feedback signals. As in contact mode, information is obtained using such feedback signals, which typically are collected, stored, and used as data to characterize the sample.
Regardless of their mode of operation, AFMs can obtain resolution down to the atomic level on a wide variety of insulating or conductive surfaces in air, liquid or vacuum. Such AFMs typically do so using piezoelectric scanners, optical lever deflection detectors, and very small cantilevers fabricated using, for example, photolithographic techniques. Because of their resolution and versatility, AFMs are important measurement devices used in many diverse fields ranging from semiconductor manufacturing to biological research.
There are many ways to control the position and movement of the sensor tip, most of which involve actuating the cantilever in some way. This typically is done using one or more drives, typically configured as actuation elements such as a piezo actuator attached to the substrate or base of the probe holder device. The cantilever drive, in turn, is further attached to a translational scanner which displace the probe in z and xy directions. A displacement control signal is provided to each of the piezo elements as needed to displace the cantilever and/or the sample, such as to produce the relative movement between the probe and sample to scan the sample in the desired manner. If desired, more than one drive signal can be inputted to a particular actuation element and/or the inputted drive signal can be made up of multiple components. In any event, a desirably configured drive signal provided to an appropriate drive(s) can be used to oscillate the probe tip where such oscillation is desired.
The drive signal typically is an electric signal. The drive signal is generated by some other component, such as a controller or the like. The resultant drive signal can be based on feedback of some sort. For example, it is common for a drive signal used in exciting a probe into oscillation to be based at least in part on feedback of some form. For example, one type of commonly used feedback is obtained from analyzing or measuring the force of the tip interacting with the sample during operation.
Depending upon the magnitude, polarity and frequency of the applied voltage of the drive signal, selective actuation of the cantilever helps locate the tip of the probe relative to the sample during operation. In addition, where it is desired to oscillate the tip, this enables the cantilever to be excited so that it and the tip will oscillate at a desired frequency or within a desired frequency range during operation.
Feedback, such as feedback relating to forces between the tip and the sample, is often used to determine how to position the cantilever. For example, where oscillating the tip, feedback relating to the interaction force between the tip and sample can be used in determining a control signal or some component thereof that is provided to one or more of the positioning devices to produce the desired oscillation and/or move the cantilever in a desired manner. The cantilever drive signal can be based on, for example, the same as, or a proportional frequency, to a resonant or natural frequency.
One commonly used type of drive is a piezoelectric drive, such as is typically employed in piezoelectric scanners. A typical piezoelectric drive includes a plate or stack of piezoelectric material located at or adjacent a fixed end of the cantilever. A drive signal, in the form of an applied voltage, is applied to the piezoelectric material to cause it to expand or contract, depending upon polarity of the applied voltage.
Because AFM's use such small cantilevers, which are roughly 1000 times smaller than the cantilevers used in more traditional cantilever measurement instruments, it has been a challenge to couple the piezoelectric actuation to excite the cantilevers. Conventionally, the piezo-stack attached to the probe device base is used to actuate the entire base, causing the cantilever to oscillate due to inertial force. The weight of the base is usually more than one-hundred thousand times than that of the cantilever, and therefore the actuation required to move the base takes significant mechanical power that causes the system to communicate significantly more mechanical energy to the environment than to the cantilever itself. The excess mechanical energy passed to the environment often results in parasitic resonances of the entire fixture.
These problems can be tolerable in certain instances, for example when the analysis is performed in air given the high mechanical Q of the cantilever. Even though the piezo drive coupled to the cantilever is mechanically “dirty,” the high Q cantilever, with Q as much as 100 to a few hundreds in air, functions as a 40 db to 60 db bandpass filter. The oscillation signal of the cantilever is therefore clean and free of parasitic resonance in most cases. However when the cantilever is placed in fluid, an environment needed, for example, when imaging biological samples, the mechanical Q factor of the cantilever drops to two to three, and all the parasitic resonances are coupled into the cantilever oscillation. Such abundant parasitic resonances in fluid are often called “forest of peaks,” referring to the resultant response (amplitude vs. frequency) curve. This is one of the major problems with AFM operation in fluid.
These problems have been relatively adequately addressed in the past by including a layer of piezoelectric material deposited on the cantilever itself to form a composite cantilever. Voltage applied to the piezoelectric layer induces strain in the piezoelectric layer, which in turn causes the cantilever to correspondingly displace. Unfortunately, fabrication of these types of composite cantilevers can be undesirably complicated and may not always work well in a fluid environment, particularly when passivation is required.
Another type of drive that has been investigated in the past is a magnetic drive. A magnetic drive utilizes a composite cantilever construction that employs a magnetic layer disposed on the cantilever in place of the piezoelectric layer. A magnetic drive coil is located in the vicinity of the magnetic layer on the cantilever. Electric current introduced to the magnetic drive coil generates a magnetic field that acts upon the magnetic layer of the cantilever to cause the cantilever to correspondingly displace. While this type of composite cantilever requires no passivation, the magnetic drive coil inherently functions as a low pass filter, which can undesirably limit the frequency at which it can oscillate the cantilever. For example, the low pass filter characteristics of the magnetic drive coil can limit the effective cantilever oscillation frequency of magnetic drives to less than about 50 kHz or lower. In addition, this type of composite cantilever fabrication poses considerable challenges, especially with regard to large current heating of the coil, in the order of one amp. Finally, sharpness of the probe tip is often compromised as a result of the magnetic coating, which can significantly limit the applications in which this type of a composite cantilever can be used.
A further type of drive that is useful is an ultrasonic drive. An ultrasonic drive employs an ultrasonic wave generator that directs ultrasonic waves toward the cantilever in a direction normal to the cantilever and in-line with the desired direction of cantilever displacement. Ultrasonic waves have been used not only to displace the cantilever in a desired direction at a desired magnitude, they have also been employed to excite a cantilever into oscillation.
While an ultrasonic drive works with any type of cantilever and does not require any special cantilever fabrication, it poses its own unique challenges because it requires focusing of the ultrasonic waves into a beam that accurately impinges against the cantilever. Because scanning probe cantilevers are so small, it can be difficult to consistently focus the ultrasonic beam onto the cantilever resulting in inconsistent drive operation. In addition, where the cantilever is used in a fluid environment, the ultrasonic beam typically creates a high pressure gradient in the fluid, which can lead to a disadvantageous phenomenon called “acoustic streaming.” When this occurs, the high pressure gradient in the fluid causes fluid convection in the vicinity of the cantilever, which may cause the cantilever to move in a random and uncontrollable manner. Cantilever force sensing can be compromised, which in turn can adversely impact cantilever control, all of which can render the results obtained unusable.
Other arrangements have also been employed to excite or cause a cantilever to displace, including in an oscillatory manner. One such arrangement is an electrostatic drive that electrostatically excites the cantilever using an electrically charged electrode located in the vicinity of the cantilever. This produces an attractive force that attracts the cantilever to the electrode. The frequency and magnitude of the voltage applied to the electrode can be varied to correspondingly drive the cantilever, including into oscillation.
Another such arrangement is a magnetostatic drive that employs magnetostatic excitation to drive the cantilever. A small magnet having a mass much less than that of the cantilever is attached at or adjacent the free end of the cantilever. A magnetic drive coil is placed in the vicinity of the cantilever and an electric current is applied to cause the coil to generate a magnetic field. The magnetic field acts upon the magnet on the cantilever thereby causing the cantilever to displace in concert with the magnet. The frequency and intensity of the magnetic field produced by the coil can be varied by correspondingly varying the frequency and magnitude of the applied electric current, which typically is alternating current or the like.
A still further arrangement is an induced eddy current drive. This type of drive utilizes a high frequency coil placed in the vicinity of an electrically conductive cantilever. Voltage or current is applied to the coil to cause a magnetic field to be generated that induces an eddy current in the cantilever. Because the eddy current, in turn, produces another magnetic field, the resultant interaction between it and the magnetic field generated by the coil excites the cantilever if the frequency of the magnetic field is modulated at a lower frequency so as to permit the cantilever to mechanically follow.
While several of these types of drive arrangements have proven to be very successful, improvements nonetheless remain desirable. Hence, the need has arisen for a method and arrangement for displacing the cantilever in a controllable manner which can be used alone or in combination with one or more of the aforementioned drive arrangements. The need has also arisen for a method and arrangement of providing an array of probes located sufficiently close to aid operational speed and ease of use by, for example, lessening tedious set-up procedures.